Method of treating metallic surfaces



June 24, 1969' BAUER ET AL 3,451,871

METHOD OF TREATING METALLIC SURFACES Filed May 25, 1965 Sheet of 2 FIG.1

/3 IL IL I l m 20 l I INYENTORS T2, Hanna 13am My. Javier Q TTORNEY June24, 1969 H. BAUER ET AL 3,451,871

. METHOD OF TREATING METALLIC SURFACES Filed May 25, 1965 Sheet 2 of 2FIG. 4

- F/ [Cr/6. 6 a

HEXE LZE United States Patent U.S. Cl. 156-244 13 Claims ABSTRACT OF ITHE DISCLOSURE A method of treating at least one surface of a metalmember, such as for instance an elongated sheet metal member, so as toimprove the adherence of a cover layer thereto, in which the surface issubjected to the action of a high voltage, high frequency alternatingfield Vesta-blished between the member and a rotating electrode spacedtherefrom.

The present invention relates to a method of treating metallic surfacesand, more particularly, the present in- ;vention is concerned with anelectric pretreatment of metallic surfaces for the purpose of increasingthe surface activity thereof with respect to the adherence of coverlayers, for instance lacquers, printing inks, adhesives, or syntheticplasticvmaterial which may be applied to such metallic surfaces.

.The metallic surfaces which are to be treated according to thepresentinvention may, prior, thereto, be subjected to a pre-treatment bychemical or physical methods for the purpose of cleaning the surfacesofv impurities adhering thereto. For instance, sheets, bands or foils ofaluminum or aluminum alloys maybe subjected to soft annealing in orderto remove fat therefrom, or the metallic surface may be cleanedmechanically by brushing, or chemically by applying primers or primingsubstances which will improve the adherence of subsequently appliedcover layers and the like, or by treatment with detergents orlacquer-like bodies. All of these treatments have the purpose ofcleaning a metallic surface and/or, at least to a limited extent,activating thesame so that a firmer adherence of the subsequentlyapplied cover layer or the like can be achieved.

Treatment according to the present invention maybe carried out inconnection with metallic surfaces which have thus been pretreated butalso in connection with metallic surfaces which have thus been'pretreated but also in connection with metallic surfaces which have notbeen subjected to any pretreatment. I It is an object of the presentinvention to provide, a

3,451,871 Patented June 24, 1969 With the above and other objects inview, the present invention contemplates a method of treating a metalsurface so as to improve the adherence of a cover layer thereto,comprising the step of subjecting hte metal surface to a high voltage,high frequency alternating field.

The present invention also includes a method of treating the surface ofa metal body so as to improve the adherence of a cover layer thereto,comprising the steps of subjecting a surface portion consistingessentially of a metal selected from the group consisting of aluminum,copper, lead and tin to a high voltage, high frequency alternating fieldof between 50,000 and 600,000 volts and between 25 and 400 kHz., andapplying a cover layer to the thus treated surface portion.

According to the present invention, the metal surface, which may or maynot have been pretreated, for instance as described above, is subjectedto a short time exposure to a high voltage, high frequency alternatingfield, whereby the metal body, the surface of which is to be treated,serves as one of the two electrodes between which the high voltage, highfrequency alternating field is formed.

The alternating high frequency field which is to be applied according tothe present invention, generally should have a voltage of between 50,000and 600,000 volts, and preferable between 400,000 and 600,000 volts, anda frequency of between 25 and 40 kHz. and preferably between 25 and 75kHz. Under these conditions, a corona effect is avoided.

It has been found that in this manner an excellent adherence can beachieved between various covering materials such as printing ink,metalizations, adhesive, etc. and the thus treated metallic surface. Forinstance, in the case of aluminum and aluminum alloy surfaces, due tothe greatly increased ability of thus treated aluminum or aluminum alloysurface to hold or adhere high polymeric synthetic materials, it ispossible to form a very firm adherence between aluminum foils andpolyolefins which are extruded onto the same (and which generally showpoor adherence to aluminum) and, on the other hand, it is possible toform, after the pretreatment of the aluminum foil in accordance with thepresent invention, cover layers of extruded polyolefins on the aluminumor the like surface at a much greater forward speed than was up to nowpossible.

By the treatment according to the present invention a continuouselectric discharge takes place between the surface of the metal to betreated and the electrodes which are spaced therefrom and this willactivate the treated metal surface to such an extent that non-metallicas well as metallic materials can strongly and permanently adhere to thethus treated metal surface.

A further advantage of the activation of the metal surface according tothe present invention is found, for inmethod of treating metallicsurfaces which will improve the adherence of covering materials such 'aslacquers,

printing inks, adhesives or plastic materials such as synthetic plasticmaterials, for instance polyethylene films and the'like thereto andwhich will also improve the adherence of metal applied to the thustreated surface.

:iIt is a further object of the present invention to pro- 7 stance, inapplying high pressure polyethylene extrusion layers to the activatedmetal surface. It is possible, to extrudethe polyethylene atconsiderably lower temperatures than were required up to now in order toobtain a firm adherence to the metal surface. For instance, it has beenfound in many cases that the extrusion temperature of the polyethylenemay be reduced by between 20 and 30 C. below the extrusion temperaturewhich would be required in order to adhere the extruded polyethylene toa similar metal surface which has not been subjected to a high voltage,high frequency field in accordance with the present invention.Nevertheless, the adherence of the polyethylene which had been extrudedat such lower temperature to the metal surface, such as for instance .an

aluminum surface which had been subjected to the high voltage highfrequency treatment in accordance with the present invention, has beenfound to be equally strong as the adherence of extruded polyethylenewhich had been extruded at the higher temperature to a similar aluminum3 or the like foil which had not been subjected to the high frequencyand high voltage alternating field in accordance with the presentinvention.

The activation of the metal surface which is achieved by the highfrequency high voltage alternating field treatment goes far beyond theactivation which, for instance, is accomplished by soft annealing orotherwise removing fat and cleaning the surface of the metal and theactivation according to the present invention results in an adherence ofthe cover layer which is much stronger than could be obtained up to noweven on a completely clean metallic surface.

It may be assumed, however, without limiting the present invention toany specific theory, that by subjecting the metal surface to the highfrequency, high voltage alternating field, particularly within the abovedescribed limits of voltage and frequency, the characteristics of themetal surface will be changed in a manner which has not yet been fullydetermined.

The present invention is of particular significance with respect toapplying polyethylene layers to metal foils, such as aluminum oraluminum alloy foils, since, due to the fact that the polyethylene neednot be heated to such high temperature as was previously required, apolyethylene layer will be formed which will not only firmly adhere tothe aluminum or the like foil, but which will be, to a much greaterextent than was hitherto possible,

free of odors. This is particularly important with respect to theutilization of such polyethylene coated aluminum foils as a packingmaterial in the food industry.

. It is also frequently of great importance that by proceeding inaccordance with the present invention, it is frequently unnecessary totake steps in order to improve the ability of the metal foil to have acover layer firmly adhered thereto. When it is nevertheless desired tocarry out the preliminary steps for improving the ability of the metalfoil to adhere a cover layer thereto, for instance, when it is desiredin the case of aluminum to subject the surface to an anodizing treatmentprior to the high frequency, high voltage alternating pretreatmentaccording to the present invention, than, by combining these twotreatments, a further improvement in the ability of the metal surface toadhere a cover layer thereto will be achieved so that a coherencebetween metal surface and cover layer is obtained which is of greaterstrength than could be achieved up to now.

Generally, it is desirable to utilize for the high frequency, highvoltage alternating field treatment a current of between 25 and 75 kHz,or even higher, and of a voltage which-although in some cases might beas low as 50,000 volts, generally will be 300,000 volts or more,preferably between 400,000 and 600,000 volts. Within these limits, thedesired surface activation of the metal is obtained with a very highdegree of certainty.

According to a preferred embodiment of the present invention, it is aparticular advantage of the same that a firm adherence, or an increasein the adherence of the covering material on the metal surface can beaccomplished without requiring more or less involved pretreatment steps.Only in rare cases, when an extraordinarily high degree of cohesionbetween the metal surface and 'the covering material is required, willit be desirable to combine conventional pretreatment steps such asdescribed above, for instance defatting or other purification of a metalsurface, or anodizing, with the high frequency, high voltage alternatingtreatment according to the present" invention. The high frequency, highvoltage alternating field treatment according to the present inventioncan be carried out, for instance, on aluminum foils or other aluminumsurfaces, whereby the aluminum may be either in hard condition or insoft annealed condition.

As will be described in more detail in connection with the drawings, ithas been found advantageous to form the high frequency, high voltagealternating field between electrodes which preferably are provided withan insulating layer at the portion thereof facing the alternating field.The electrodes may be stationary or, preferably, may be formed asrotating rollers, whereby the metal sheet or foil having a surface whichis to be treated is preferably in contact with the opposite relative tothe treating electrode, and the metal sheet or foil is moved intangential direction relative to the treating electrode.

The use of a treating electrode of roller-like configuration will havethe advantage that the discharge will take place along a line across theentire width of the material, i.e. the metal sheet, foil or the like,whereby the line of discharge on the surface of the roller electrodewill continuously change its position due to rotation of the rollerelectrode. This will prevent undue heating of the roller treatingelectrode or localized over-heating of the same. Furthermore, anelectrode in the shape of a roller afiords an easy opportunity forcooling of the electrode. Preferably, the roller is supported bybearings of highly insulating material in order to avoid voltage losses.7 I

The width of the gap which is to be maintained between the metal surfacewhich is to be treated and the treating electrode will depend only to aslight degree on the electric conductivity of the dielectric mediumwithin the gap between treating electrode and metal surface, whichdielectric medium generally will be air enriched with ozone formed bythe discharge, and will primarily depend on the voltage which isapplied. Generally, the width of the gap should be between 0.3 and 10mm., and preferably between 0.5 and 3 mm.

Since the extent to which the surface characteristics of the metal withrespect to the firmness of adherence of cover layers thereto areimproved according to the present invention will depend on the type ofmetal or alloy, it is sometimes advantageous to repeat the highfrequency, high voltage alternating field treatment several times, forinstance by arranging several treating electrodes in. sequence so thatthe metal surface which is to be treated passes successively through thealternating fields created between the metal body the surface of whichis to be treated and the respective treating electrodes. On the otherhand, it may be advantageous to arrange several treating electrodes insuccession because thereby the speed of passage of the metal foil or thelike through'the treating device can be increased.

Furthermore, the metal sheet or foil which is to be treated may be movedalong such a path, or several treating electrodes may be arranged insuch a position, that not only one but both surfaces of the metal sheet,foil or the like will be exposed to the high frequency, high voltagealternating field.

Preferably, the metal sheets or foils or semifinished products, forinstance metal tubes, which are to be subjected to a high frequency,high voltage alternating field in accordance with the present invention,are treated in conventional production lengths which, for instance inthe case of metal sheets or foils, may be from several hundred toseveral thousand meters, because in this manner it is possible tooperate a substantially continuous process. In the case of shorter metalbodies, for instance cut sheets, the alternating field treatment can beeither carried out intermittently, separately for each cut sheet, or theindividual cut sheets are arranged in sequence with very little distancebetween adjacent individual cut sheets so that the voltage of thetreating electrode can be maintained permanently, i.e. for the entirelength of time during which successive cut sheets are passed through thetreating device.

It is advisable to form the guide rollers for the sheets, foils, tubesor the like which are located above or below these metal bodies in thevicinity of the treating electrode, of non-conductive materials such asrubber or the like in order to avoid flash-over of electric current inthe vicinity of the treating electrode. Provided that the voltage issufiiciently high, the width of the sheet or the like which is to betreated can be varied as desired without marked 15 influence on theimproved surface characteristics which are achieved by the treatment. Itis of course desirable that the treating electrode has at least the samewidth as the metal sheet or the like which is to be treated.

The method of the present invention is suitable not only for thetreating of the surfaces of metal sheets and foils, but also fortreating the metal surfaces of differently shaped bodies, for instancepipes, whereby the shape of the treating electrode must be adjusted tothat of the surface which is to be treated so that a gap ofsubstantially even width throughout is formed there'between.

The high frequency, high voltage alternating field treatment of themetal surface in accordance with the present invention may be carriedout immediately prior to applying a cover layer to the thus treatedmetal surface, for instance by extrusion, printing, lining or coating indifferent manners. However, it is also possible to store the metalbodies, for instance sheets or foils which have been exposed to thealternating field in accordance with the present invention, and to carryout the subsequent application of a cover layer or the like bylacquering, printing, extrusion application of a polyolefin or the like,etc., at some later date.

As described above, the counter electrode to the treating electrode isformed by the metal body the surface of which is to be subjected to thehigh voltage, high frequency alternating field.

There is a certain. dependency between the voltage and the length oftime for which the alternating field is to be applied. In certain cases,for instance, when operating with an alternating field having a voltageof 600,000 volts, 1

the improvement in the surface characteristics of an aluminum foil forthe purpose of adhering high pressure polyethylene thereto, will beachieved by passing the aluminum foil or the like through the highfrequency, high voltage alternating field at speeds of up to 200 metersper minute. When the voltage is reduced, the speed of passage ofthemetal surface through the high frequency field must be correspondinglyreduced. In accordance with the foregoing, for instance, optimum surfacecharacteristics will be achieved with the same aluminum surface andunder otherwise similar conditions but at a voltage of only 500,- 000volts if the speed of passage of the foil through the alternating fieldis reduced to 160 meters per minute.

Generally, aluminum foils having a purity of at least 98.5% andincluding the conventional impurities such as iron, silicon, copper,etc. in accordance with the ASTM norms will give good results, as wellas aluminum alloys of the type AlMn (3003), AlMgMn (3004), AlMg 3(5052), AlMg 5 (5056), AlMnCuMg (2024), AlMgSi (6066), A1990 (1100), A199.5 (1050), A1 99.7 (1070), A1 99.9 (1090), Al 99.99 (1099 Similarresults are achieved by treating metal surfaces other than aluminum oraluminum alloys, particularly metal surfaces which consist essentiallyof copper, lead and tin, including tinned lead and tinned strip steel.

The greater the length of the treating electrode, or the greater thewidth of the metal sheet or fo'il which is to be treated, the highershould be the voltage, thus, in the case of treating electrodes having alength of 3 meters, it is desirable to apply 600,000 volts.Theoretically, it is possible to subject sheets of foil of any desiredwidth to the alternating field treatment accord-ing to the presentinvention by arranging a plurality of electrodes, each having a lengthof for instance about 3 meters, next to each other with slightlyoverlapping end portions, however, for all practical purposes it ishardly necessary to treat sheets or foils having a width of more than 3meters and thus, it is generally possible to produce the requiredalternating field with a single treating electrode.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings, inwhich:

FIG. 1 is a diagrammatic view of an extrusion arrangement according tothe present invention for applying polyolefins, for instance highpressure polyethylene onto carrier webs such as metal foils,particularly aluminum foils and compound foils 'such as paper-aluminumfoils and paper-aluminum-synthetic material foils;

FIG. 2 is a diagrammatic view of an arrangement according to thepresentinvention for applying lacquer or baking enamel to a metal sheet;

FIG. 3 is a diagrammatic view station for subjecting a metal and highvoltage alternating air cooling device;

FIGS. 4 and 5 are diagrammatic views of arrangeof an electric treatingsurface to a high frequency field, including a compressed ments forsubjecting a metal foil or the like to two suc- Although it is possibleto store metal foilsand the like, the surface of which has been treatedaccording to the present invention, and to apply a cover layer theretoafter such storage, and still achieve superior adherence,

generally, it is desirable to carry out the adherence of the tainedmetal surface of improved surface characteristics and guide rollers ofthe conveying equipment for the metal sheet, foil or the like. 7

When it is desired to pretreat the aluminum foil by soft annealing,prior to subject-ing the same to the alternating field treatment of thepresent invention, such soft annealing preferably is carried out attemperatures between about 450 and 550 C., depending on the specificcomposition of the aluminum or aluminum alloy.

, been pretreated andactivated cessively arranged high frequency, ingfields; and

FIGS. 6 and 6a are respectively schematic front and side views of atreating device for applying a high frequency, high voltage alternatingfield to a metal sheet, including a stationary treating electrode.

Referring now to the drawing and particularly to FIG. 1, referencenumeral 4 denotes the arrangement for successively applying two highvoltage, high frequency alternating fields to a metal foil such as analuminum or aluminum alloy foil. The two rollers 4 beneath continuousmetal sheet or foil30 are guide rollers and consist of anozone-resistantinsulating material, for instance silicone rubber.Rollers 4 serve for adjusting the exact width of the gap between theupper surface of aluminum sheet 30 and treating electrodes 4". Treatingelectrodes 4" are roller electrodes of the typeillustrated in moredetail in FIGS. 4, '5 and 6. v

Starting now with the left hand portion of FIG. 1, it will be seen thatthe aluminum foil or sheet 30 passes from an unwinding station 1through. alacquer applying station 2 and a drying channel 3. to thealternating field treating station 4. The aluminum sheet whichthus haspasses then between pressure roller 5 andcooling roller 8, and duringthe passage between these two rollers, molten high pressure polyethylenewhich has been molten in extrusionscrew conveyor 6 is applied to theactivated surface'of aluminum high voltage alternat sheet 30 throughslot-shaped nozzle 7. The thus formed composite sheet with thepolyethylene firmly adhering to thealuminumiswthen wound at coil winderorw-inding station -9. I

According tothe present inventionit is also possible, as will bedescribed in some. of. the examples herein, to eliminate the primingstations 2 and .3 and to pass the aluminum sheet or the like directlyfrom unwinding device 1 to the alternating field treating arrangement 4.

However, by using the entire device as illustrating in FIG. 1, includingthe priming arrangement, the improvement in adherence between thealuminum surface and the extruded polyethylene which is achieved byapplication of the chemical primer at the lacquering arrangement 2 andevaporation of the solvent for the lacquer in drying channel 3, isfurther increased by the application of the alternating field intreating device -4 so that the adherence of the polyethylene to thealuminum sheet will be much stronger than could be achieved by onlypriming the aluminum sheet without application of the alternating field.

A chemical primer which may be advantageously used by proceeding inaccordance with FIG. 1, is shellac which must be free of wax and whichis preferably applied in an amount of 0.01 gram of shellac per squaremeter of aluminum surface.

FIG. 2 shows unwinding device 10 from which the aluminum band or thelike passes through an alternating field pretrea'ting station 11including treating electrode rollers 11' located beneath aluminum sheet30 and guide rollers 11" formed of insulating material and serving formaintaining the desired gap between the metal sheet 30 and treatingrollers 11'. After being exposed to the alternating field in treatingstation 11, aluminum sheet 30 passes through a conventional lacquerapplication device 12 and a drying or baking channel 13, and from thereto coil forming device 14.

According to FIG. 3, the metal foil 21 is connected to earth by means ofmetallic guide rollers 17. The treating station for applying thealternating field comprises guide roller 16 formed of ozone resistantinsulating material such as silicone rubber or the like, and treatingelectrode 15, the details of which are described in FIGS. 4, and 6. Thedistance between treating electrode 15 and the surface of foil 21 isshown in an exaggerated manner, in fact, and as described further above,the gap between the free surface of foil 21 and treating roller 15should not exceed mm., and preferably will be between 0.5 and 3 mm.

Guide roller 16 is supported by sliding bearings so as to permitadjustment of the gap between guide roller 16 and electrode roller 15.At low forward movement of metal sheet or foil 21, such as 5 meters perminute and less and high intensity of the alternating leld, it mayhappen that the surface of treating roller or electrode will be undulyheated. Excessive working temperatures at the surface of electrode 15would cause accelerated wear and tear of the same and could lead toshort circuits. Generally, such low forward speed of the aluminum foilor the like will not be encountered with the exception of a combinationof the alternating field treating device with a relatively old fashionedarrangement for applying a baking enamel. Whenever the-operatingtemperature of the roller electrode 15 rises above a desired upperlimit, it is possible to cool roller electrode 15 by applying compressedair through nozzles 18. However, if compressed air emanates from nozzles18 at a very high speed, then an undesirable fluttering of a relativelythin metal foil could occur between rollers 16 and 17. Thus there arecertain practical limitations with respect to the speed at whichcompressed air may be released through nozzles 18. However, by coolingthe compressed air, it is possible to achieve the desired cooling ofroller 15 even without excessive speed of the compressed air. H

Treating electrode 15 is connected to high frequency transformer 19, theother terminal of which is grounded. The transformer receives theprimary voltage from high frequency generator 20. I Suitable apparatusof this type, is available for instance under the trade name VetaphoneTreater T300/ 3. FIGS. 4 and 5 are diagrammatic illustrations ofarrangements for exposing the-metal sheet, such as an aluminum foil 21to two successively arranged high frequency, high voltage alternatingfields.

Guide rollers 22 are formed of insulating material and are turnablyarranged in sliding bearings so that the gap between foil 21 and thetreating rollers can be adjusted. The treating electrodes according toFIGS. 4 and 5 are turnably supported by bearings made of insulatingmaterial. The treating electrodes or rollers are rotated by suitabledrive means, for instance V-belts and it is not necessary that thesurface speed of the electrode rollers is equal to the forward speed ofmetal foil 21. When operating with relatively small forward speeds ofmetal foil 21, it has been found advantageous to permit the electroderollers to rotate at somewhat greater surface speed than wouldcorrespond to the forward speed of foil 21 since thereby a coolingeffect is achieved on the surface of the electrode rollers.

In order to achieve an even distribution of the voltage applied to thetreating electrode, the same must consist of a body of high electricresistance and simultaneously must be arranged as condenser in anoscillatory circuit, whereby the metal foil 21 forms the zero potential.

The treating electrodes or rollers shown diagrammatically in FIGS. 4 and5 consist in their innermost portion of a steel core 26 to which thevoltage is applied. Metal foil 25, preferably consisting of aluminum,serves for even distribution of the charge. The entire outer surface ofmetal foil 25 is covered with a layer 24 consisting of metal powder orof a pulverulent semi-conductor, preferably graphite. By changing thethickness of layer 24 or by choosing as the material thereof more orless highly conductive pulverulent material, it is possible to changeand control the resistance values and thus the condenser effect of theentire treating electrode. The outermost layer 23 of the roller ortreating electrode consists of a heatresistant insulating material, forinstance of the material known under the trade name Pertinax (PhenolicResin).

According to FIG. 41, both treating rollers will affect the same face ofmetal sheet or foil 21, while FIG. 5

' shows an arrangement whereby the successively arranged treatingrollers will affect opposite faces of foil 21, respectively, so thatboth sides of foil 21 will be treated and improved with respect to thesurface characteristics in connection with the subsequent adherence of acover layer thereto.

FIG. 6 is a schematic illustration of a stationary treating electrode.It is characteristic for the use of stationary treating electrodes thata very highly heat resistant insulating material must be employed.

Thus, for treating electrodes of this type, Pyrex glass layer 28 hasbeen found to form a suitable insulating layer, since Pyrex glass canwithstand temperatures up to 200. The inner surface of insulating layer28 is preferably completely covered with an aluminum foil 29, in orderto assure even voltage distribution over the entire electrode. The outerelectrode surface is formed by open loops 27 of electric resistancewire, for instance of a manganese or Konstantan (alloy: 54% Cu; 45% Ni;5%

- Mn) wire of 0.1 mm. diameter. The individual wire loops 27 which areinterrupted as illustrated, serve primarily for achieving an evendistribution of the electric discharge. The metal surface which is to betreated, as well as the turnably supported electrodes are past at thedesired distance tangentially to the surface of the stationaryelectrode, whereby the gap between the stationary treating electrode andthe metal sheet or foil 21 can be adjusted by means of an insulatedguide roller.

In accordance with the method of the present invention A and with thedevices diagrammatically illustrated in the drawing and describedhereinabove, it is always possible to achieve an improvement of thesurface characteristics of a metal sheet or other metal body which issubjected to the high frequency, high voltage alternating field,provided that the body which is to be thus treated is either a metallicconductor, such as a metal sheet or foil, or a composite foil whichincludes at least one conductive metal layer. 7

It is known to improve the adherence characteristics of polyolefinfoils, cellophane and various paper and cardboard types by applicationof a high frequency alternating field which will result in coronadischarges. In this manner, it is possible, for instance, to change thesurface of a polyolefin layer so as to make it possible to imprint thesame. However, this effect cannot be achieved by means of theconventional electric pretreatment of these carrier materials if thesame are already connected to a metal web, for instance an aluminumfoil. Similarly, the conventional method is inoperative in the case of anon conductive carrier sheet or web, for instance of paper which iscoated with a synthetic material in which a metal powder, for instancealuminum bronze is incorporated. Due to the applied high voltages, thesemetal powders will cause the entire layer to become conductive and thismakes it impossible to utilize the conventional method which isconnected with corona discharges.

g The high frequency, high voltage treatment in accordance with thepresent invention overcomes the above discussed difliculties and permitsnot only the treatment of exposed metallic surfaces but also ofnon-metallic surfaces provided that the same are included in a compositestructure which includes a metallic layer. The effect achieved therebygives results which somewhat correspond to those which are obtained bymeans of the corona elfect in the case of non-conductive materials orcomposite layers which are free of conductive components.

While the present invention is described herein primarily with respectto the treatment of aluminum and aluminum alloy foils, it is emphasizedthat the present method is also suitable for improving the surfacecharacteristics with respect to adherence of cover layers to other metalbodies such as sheets or foils of steel, tinned steel, zinc-coatedsteel, copper, lead, tinned lead and tin.

Foils of the above-mentioned metals and metal combinations weresubjected to the high frequency, high voltage alternating field asdescribed above, and then in an extrusion device coated with about 50grams per square meter of high pressure polyethylene. It was found inall cases that the adherence of the polyethylene layer to the metalsurface was much stronger when the metal surface had been subjected,prior to application of the polyethylene, to the alternating fieldtreatment according to the present invention. In these cases, thealternating field pretreatment was generally carried out at a distanceof about 1 meter from the extrusion nozzle for the polyethylene. Thetemperature of the polyethylene at the extrusion nozzle was about 280C., and an alternating field of. 600,000 volts and -50 kHz. was applied.The gap between the metal surface which was to be treated and thetreating electrode was maintained at 2 mm.

Lead and tin sheets were passed through the alternating field treatingdevice at a speed of meters per minute and the other metals mentionedabove at a speed of 60 meters per minute.

Metal sheets of commercial quality were used and it was found that thepurity or the analysis of the material was nearly without any influenceon the effect of the high voltage, high frequency alternating fieldtreatment. Generally, the width of. the thus treated metal sheets ,wasbetween 30 and 80 cm.

The following examples are given as illustrative only of the presentinvention without, however, limiting the invention to the specificdetails of the examples.

EXAMPLE 1 A soft annealed aluminum foil of 0.015 mm. thickness and awidth of 1,000 mm. is coated in a conventional extrusion device with alayer of high pressure polyethylene. If the aluminum foil has not beensubjected to any kind of" surface pretreatment, it will be found that ata forward speed of 30 meters per minute or more the adherence betweenaluminum surface and polyethylene, by application of the polyethylene ata nozzle temperature of about 280 C., will be unsatisfactory.

However, by pretreating the aluminum foil surface by subjecting the sameto an alternating electric field of 600,000 volts and kHz. at a distanceof about 1 meter before the extrusion nozzle, the adherence between thehigh pressure polyethylene and the aluminum surface was found to be sostrong that a high pressure polyethylene layer of 0.050 mm. thicknesscould be applied even at a forward speed of the aluminum foil of 60meters per minute and more. The thus formed compositealuminumpolyethyelne foil could not be separated into its componentswithout tearing of the polyethylene film. The distance between thetreating electrode roller and the aluminum surface, or the gaptherebetween was maintained at 1.2 mm. 1

For the purpose ofthis example, a roller electrode arrangement producedby the Deutsche Vetaphone G.m.b.H. Hamburg, Model No. T300/3 was used.

EXAMPLE 2 Substantially in the manner illustrated in FIG. 1, however,without any priming steps, an aluminum band having a thickness of 0.050mm. and a width of 800 mm. was subjected to the alternating fieldtreatment described in Example 1 at a forward speed of 25 meters perminute, and without being prior thereto subjected to any chemical ormechanical or other pretreatment. The thus surface activated sheet wasthen formed into a coil and stored for 15 days. Thereafter, the sheetwas uncoiled and to the activated surface thereof high pressurepolyethylene was applied in a conventional extrusion arrangement at atemperature of the polyethylene of 280 C. at the nozzle orifice and in athickness of 70 grams of polyethylene per square meter. Very strongadherence between the polyethylene and aluminum layers was observedwhich far surpassed that which could be achieved by conventionalmethods.

of intermediate hardness, having a thickness of 0.2 mm. and a width of600 mm. was combined with a layer of baking enamel without anypretreatment of the aluminum band. The forward speed of the aluminumband was 5 meters per minute, the baking temperature 230 C. and thethickness of the lacquer film 5 grams per square meter.

By proceeding in this manner, only insufficient adherence of the lacquerto the aluminum band could be achieved. However, upon first subjectingthe surface of the aluminum band to the alternating field treatmentdescribed in Example 1, with a distance between the treating electrodeand the lacquer applying and baking device of 1.5 meter, it was foundthat thebaked enamel layer adhered so strongly to the aluminum surfacethat even difficult deformation operations such as deep drawing of thelacquered aluminum sheet could be carried out without weakening the bondbetween the baked enamel and the aluminum surface.

EXAMPLE 4 meter from the nozzle of the polyethylene extrusion de-EXAMPLE By adhering to an aluminum foil as received from the rollingmill and having a thickness of 0.015 mm. and a width of 750 mm. apolypropylene foil of 0.050 mm. under interposition of a 2-componentadhesive on the basis Desmophen-Desmodur, it will be found that theadherence of the adhesive to the aluminum surface, and thus theadherence of the polypropylene foil is unsatisfactory.

If, however, the surface of the aluminum foil is pretreated by beingexposed to an alternating field as described in Example 4, with adistance of 2 meters between the treating electrode and the device forapplying the adhesive, and subsequent adherence of the polypropylenefoil to the adhesive covered aluminum surface, the desired high strengthof adherence between the aluminum foil and the polypropylene foil wasachieved.

EXAMPLE 6 An aluminum foil as received from the rolling mill (i.e.without any pretreatment) having a thickness of 0.009 mm. and a width of1,400 mm., was rewound at a speed of 200 meters per minute andsimultaneously subjected to the alternating field treatment according tothe present invention substantially as described in Examples 4 and 5,however, so that both sides of the aluminum foil were exposed to thehigh frequency, high voltage alternating electric field, substantiallyas illustrated in FIG. 5.

The improvement in the surface characteristics of both surfaces of thefoil which was achieved in this manner made it possible, without furtherpretreating, to imprint the aluminum foil, to apply baking enamelthereto and to apply non-metallic foils thereto, for instance apolypropylene foil in the manner described in Example 5.

EXAMPLE 7 A laminated foil consisting of a soft annealed aluminum foilhaving a purity of 99.5, a thickness of 0.012 and a width of 1,000 mm.,and glued thereto a kraft paper having the weight of 60 grams per squaremeter, was extrusion coated at the free face of the paper layer withhigh pressure polyethylene in a thickness of 50 grams per square meter.

At a forward speed of the composite foil of 200 meters per minute and atemperature of the polyethylene of about 280 C. at the nozzle orifice,the adherence between the paper surface and the extruded polyethylenewas unsatisfactory.

However, by pretreating the composite paper-aluminum foil in a devicecorresponding to that schematically illustrated in FIG. 1 with analternating field of 500,000 volts and 50 kHz., and with a distance ofabout 1 meter between the treating electrode and the extrusion nozzlefor the polyethylene, and while maintaining a gap of about 1.5 mm.between the free surface of the paper and the turnably supportedtreating electrode roller, it was found that the high pressurepolyethylene adhered very strongly to the paper surface. It was notpossible to tear off the polyethylene film without at the same time alsotearing the entire composite or laminated foil structure.

EXAMPLE 8 A soft annealed aluminum foil having a purity of 99.5 athickness of 0.009 mm. and a width of 600 mm. is combined by means of alacquer adhesive with a cellophane foil weighing 35 grams per squaremeter. The thus formed composite foil is coated at the free aluminumface thereof in an extrusion device with 30 grams per square meter ofhigh pressure polyethylene.

At a forward speed of the aluminum-cellophane foil the polyethylene andaluminum surface is entirely insufficient. Application of a chemicalprimer, namely waxfree shellac in a quantity of 0.05 grams per squaremeter, in a device such as illustrated in FIG. 1 and identified thereinwith reference numerals 2 and 3, did not suffice for sufficientlyimproving adherence betweenthe aluminum and polyethylene surfaces atforward speeds of the aluminum-cellophane foil of 60 meters per minuteand more.

Upon applying in addition to the shellac primer also the alternatingfield pretreatment of the aluminum surface in accordance with thepresent invention, utilizing two electrodes arranged as illustrated inFIG. 4 and applying 600,000 volts and 60 kHz., it was found that theadherence between the high pressure polyethylene and the aluminumsurface became so strong that the polyethylene could no longer beremoved from the aluminum surface without destruction. Even byincreasing the forward speed of the foil through the alternating fieldand through the polyethylene extrusion device to 150 meters per minute,the same high degree of adherence between polyethylene and aluminumsurface was obtained.

The two treating electrodes were located 70 and cm. distant,respectively, from the extrusion nozzle and the gap between the aluminumsurface and the turnably supported treating roller electrodes amountedto 1.2 mm.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can by applying current knowledgereadily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be secured by Letters Patent is:

1. A method of treating at least one surface of a metal member so as toimprove the adherence of a cover layer thereto, comprising the steps ofsubjecting said surface to the action of a high voltage, high frequencyalternating field by passing said member in one direction closelyadjacent but spaced from an electrode and by connecting said member ascounter pole to said electrode; and rotating said electrode about anaxis extending transverse to said one direction.

2. A method as defined in claim 1, wherein said metal member is anelongated sheet metal member moved in longitudinal direction past saidelectrode.

3. A method as defined in claim 1, wherein said alternating field has avoltage of between 50,000 and 600,000 volts and a frequency between 25and 400 kHz.

4. A method as defined in claim 1 and including the step of cooling saidelectrode.

5. A method as defined in claim 4, wherein said metal member issubjected to the action of at least two high voltage, high frequencyalternating fields spaced in said one direction from each other.

6. A method as defined in claim 4, wherein said metal member issubjected at opposite surfaces to the action of high voltage, highfrequency alternating fields.

7. A method as defined in claim 1, and extruding onto the thus treatedsurface a layer of polyethylene.

8. A method as defined in claim 1 and adhering to the thus treatedsurface a sheet of polypropylene.

9. A method as defined in claim 1 and applying printing ink to the thustreated surface.

10. A method as defined in claim 1 and applying a lacquer to the thustreated surface.

-11. A method as defined in claim 1 and applying enamel to the thustreated surface.

12. A method as defined in claim 1 and extruding a layer of anextrudable synthetic plastic material to the thus treated surface.

References Cited UNITED STATES PATENTS 3,205,094 9/1965 Erlandson118-620 X 3,281,347 10/1966 Winder 25049.5 X 2,522,082 9/ 1950 Arnold11793.1 X

14 2,867,912 1/1959 Kritchever 156272 X 3,037,886 5/1962 Ryznar 1117-49X 3,323,965 5/1967 Hanle et a] l56--244 EARL M. BERGERT, PrimaryExaminer. T. R. SAVOIE, Assistant Examiner.

US. Cl. X.R. 117-49, 93.1; 156--272; 204-440; 25049.5

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.3,451,871 June 24, 1969 Heinrich Bauer et a1.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 7, "Dessau VetaphoneGesellschaft" should read Deutsche Vetaphone Gesellschaft Signed andsealed this 21st day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Commissioner of Patents Attesting Officer WILLIAM E. SCHUYLER, JR.

